Did you update from Wildfire 4.0?
Then the new (since Creo Elements/Pro 5.0) default for the selection filter (All) in combination with preselection highlighting might surprise you by causing a lot of extra work for the graphics with large assemblies/parts, as long as the mouse is hovering over geometry.
If you move the mouse focus away from geometry, it should be back to normal speed. (or if you change to a more moderate filter or switch off preselection highlighting)
Otherwise this is something else.